Technology of producing a transportation synthetic oil such as gasoline or diesel from a gas mixture of hydrogen and carbon monoxide resulting from gasification of coal or reforming of natural gas is essential for compensating for the future depletion of oil reserves, and has been thoroughly developed based on the Fischer-Tropsch (F-T) reaction well-known from the 1920s.
Fischer-Tropsch reaction is carried out mainly using an iron- or cobalt-based catalyst. Although an iron-based catalyst has been initially mainly utilized, the use of a cobalt-based catalyst is currently preferable in order to increase production of liquid fuel or wax and to raise the conversion efficiency.
An iron-based catalyst is the most inexpensive among Fischer-Tropsch catalysts, and has low methane production at high temperature and high selectivity of olefin among hydrocarbons, and a product obtained thereby may be used not only as a fuel but also as a material for the chemical industry, such as light olefin or alpha-olefin. Furthermore, in addition to the hydrocarbons, large amounts of byproducts such as alcohols, aldehydes or ketones are formed. Moreover, a commercial low-temperature Fischer-Tropsch iron-based catalyst for main use in production of wax, available from Sasol, is manufactured via precipitation using a SiO2 binder with the inclusion of copper (Cu) and potassium (K) as promoter components. Also, a high-temperature Fischer-Tropsch catalyst available from Sasol is manufactured by melting magnetite, K, alumina, and MgO.
On the other hand, a cobalt-based catalyst is undesirably expensive at over 200 times the price of the iron (Fe) catalyst, but is advantageous in terms of high activity, long lifetime, low CO2 production and high liquid paraffin-based hydrocarbon production yield. However, this catalyst is problematic because CH4 is produced in a large amount at high temperature, and is thus merely utilized as a low-temperature catalyst. Due to the use of expensive cobalt, it has to be well dispersed on a stable support having a large surface area such as alumina, silica or titania, and a precious metal promoter such as Pt, Ru or Re is further added in a small amount.
In regard to the Fischer-Tropsch synthesis reaction using a cobalt catalyst, U.S. Pat. No. 4,605,680 discloses preparation of a cobalt catalyst that is supported on γ-alumina or η-alumina and is activated with an oxide of Group IIIB or IVB metal, and U.S. Pat. No. 4,717,702 discloses preparation of a cobalt catalyst having high dispersibility of cobalt particles with a small particle size using an infiltration solution composed of an organic solvent.
Also, U.S. Pat. No. 6,130,184 discloses a cobalt catalyst having high activity via modification of a catalyst precursor and a carrier precursor, and U.S. Pat. Nos. 6,537,945 and 6,740,621 disclose catalysts having improved thermal stability and wear resistance, respectively.
In regard to the reactor for Fischer-Tropsch synthesis reaction, for a slurry reactor, U.S. Pat. Nos. 5,422,375 and 5,599,849 disclose an inner filter for catalyst separation and U.S. Pat. Nos. 5,157,054 and 5,348,982 disclose mixing of a reactant and a catalyst, and for a fixed-bed reactor, U.S. Pat. No. 6,211,255 discloses a fixed-bed reactor filled with a monolith catalyst to improve material transfer properties of a reactant and a product in the reactor, and Korean Patent Application Publication Nos. 2008-0060739 and 2009-0037089 disclose a fixed-bed reactor with a metal structure catalyst to improve material transfer properties and heat transfer properties in the reactor. Furthermore, U.S. Pat. No. 7,984,180 discloses a microchannel reactor using a cobalt catalyst for effective control of reaction heat.
However, the prior cobalt catalysts in powder or particle form as above make it very difficult to control the reaction temperature due to the severe exothermic reaction during Fischer-Tropsch synthesis reaction. Particularly upon reaction on a large scale, heat is excessively generated in the reactor and thereby the catalyst may become deactivated and also undesired side-reactions may occur.
With the goal of overcoming such problems, an inactive filler such as glass beads for preventing heat from excessively locally occurring is loaded in a reactor together with a catalyst in powder or pellet form, or any type of catalyst support including monolith or metal foam is utilized (M. Montes et al., Chemical Engineering Journal, 2011, 167, 536). Especially, when a metal foam structure in a three-dimensional shape is used as a support for a catalyst powder, it may exhibit high thermal conductivity compared to glass beads or other metal oxide beads, and may thus easily transfer heat generated in the catalyst bed during the reaction, and is also favorable for diffusion of a reactive gas to the catalyst. Conventional coating techniques of metal foam with catalyst particles are limited to wash-coating or dip-coating in such a manner that a series of procedures of immersing a metal foam in a prepared catalyst sol, taking it out of the sol and then drying it are repeated.
However, such conventional catalyst preparation methods are disadvantageous because the catalyst coating is still thick and thus problems of deactivation of the catalyst due to heat generated upon Fischer-Tropsch reaction and undesired side-reaction problems cannot be basically solved.
Attributed to difficulties in the temperature control in the reactor, a variety of products including gas products such as CH4, CO2, etc., and liquid products such as gasoline, diesel and wax may result, making it difficult to achieve selective production. Specifically, when a cobalt-based catalyst is used for Fischer-Tropsch synthesis reaction, a hydrocarbon having a long chain with 20 or more carbon atoms, namely, a wax material, may be formed. For this reason, a synthetic fuel such as wax manufactured via the Fischer-Tropsch reaction cannot be directly used as a commercial fuel for vehicles, and thus has to be converted into a commercial fuel such as diesel, which is undesirable.
Hence, there is required a direct production process that is able to selectively produce a synthetic oil such as liquid gasoline or diesel without formation of wax after Fischer-Tropsch reaction.